1. Field of the Invention
The present invention relates to a keyboard circuit and a method for detecting a keyboard circuit.
2. Related Art
Conventionally, for electric music instruments such as an electric piano, an electric organ, and the like, a switch matrix is configured which arranges a plurality of switches in a matrix form that turn ON/OFF according to an operation on a keyboard. The electric music instruments detect an operation on a keyboard by scanning this switch matrix periodically (for example, Japanese Unexamined Patent Application, Publication No. 2011-13259).
More specifically, among such electric music instruments, for an electric piano having 88 keys, two switches having different ON positions are arranged for each key in order to detect the intensity of a musical performance. Then, the electric piano measures the difference in the time at which these switches turn ON, and measures key-pressing speed on the keyboard based on the measured values. Therefore, 176 pieces of switch matrices (88 keys×2 switches) are arranged for the electric piano.
However, the number of wires on a substrate to which such conventional switch matrices are mounted (hereinafter, referred to as a “keyboard switch substrate”) approaches the limit of the capacity thereof. Therefore, a keyboard switch substrate has been desired that can significantly reduce the number of wires in a keyboard switch substrate compared to conventional ones and thus allows for wiring on a single-sided substrate. This matter is described in detail in the following with reference to FIGS. 9 and 10.
FIG. 9 is an equivalent circuit diagram illustrating the configuration of a conventional switch matrix.
As illustrated in FIG. 9, in the conventional switch matrix, eight lines of scan output signals KC0 to KC7 are arranged to cross 22 lines of input signals KI0 to KI21. A diode is connected with a switch in series near the intersection between the respective wires of a scan output signal KCi (i is any of integer values 0 to 7) and an input signal KIj (j is any of integer values 0 to 21). The diode is provided with the purpose of preventing signals from looping when a plurality of keys is simultaneously pressed.
FIG. 10 shows timing charts illustrating an outline of an operation of a conventional switch matrix.
FIG. 10 illustrates timing charts of signals flowing to each of the scan output signals KC0 to KC7 and a pre-charge signal PRC in order from the top.
In each timing chart, the horizontal axis represents time and the vertical axis represents signal level. It should be noted that explanations are provided in which each signal is a pulse signal and a signal level is explained as taking the two values of a high level (“H level”) and a low level (“L level”) in the following.
As illustrated in FIG. 10, in the conventional switch matrix, each output of the 8 scan output signals KC0 to KC7 is sequentially set to be L level in this order.
Then, with the scan output signal KCi being set to be L level, the ON/OFF states of switches connected to each wire of the scan output signal KCi and the respective wires of the 22 input signals KI0 to KI21 are detected based on each output state of the 22 input signals KI0 to KI21.
In other words, since 220N/OFF states of the switches per one scan output signal KCi are detected, 176 (8×22) ON/OFF states of the switches are detected in the conventional switch matrix having 8 scan output signals KC0 to KC7.
It should be noted that the pre-charge signal PRC is a control signal for H level that is supplied instantly from a buffer at the changing point. It is possible to correct waveform rounding and to perform high-speed scanning by supplying the pre-charge signal PRC.
In this way, on the substrate for detecting switches on which the conventional switch matrices are mounted, the wires for the 8 scan output signals KC0 to KC7 and wires for the 22 input signals KI0 to KI21, amounting to 30, are required.
However, with the structural constraints of the keyboard, it is necessary for the width of the substrate for detecting keyboard switches to be kept within a predetermined width, and the current situation is that wiring 30 signal lines almost approaches the limit of the capacity thereof.
Furthermore, the conventional switch matrix illustrated in FIGS. 9 and 10 is configured in an electric piano having two contacts (switches) for one key, i.e. an electric piano having 176 switches (88 keys×2 switches/key).
However, an electric piano having three contacts (switches) per one key, i.e. an electric piano having 264 switches (88 keys×three switches/key) has been recently produced. The conventional switch matrix illustrated in FIGS. 9 and 10 cannot be applied as it is to such an electric piano, and it is necessary to configure a switch matrix that can detect 264 switches. In this case, even if trying to realize a switch matrix with a structure similar to that of FIG. 9, a total of 41 wires for the 8 scan output signals and 33 input signals are required, a result of which the realization is extremely difficult due to the structural constraints of the keyboard as mentioned above.
In this regard, by adopting a substrate with double-sided through-holes as the substrate for detecting keyboard switches, the arrangement of 41 wires in itself becomes possible, even if the minimum width required from the structural constraint of the keyboard; however, this leads to a significant increase in cost compared to existing single-sided substrates.
Alternatively, it has also been considered to configure by connecting a plurality of substrates of the structure of FIG. 9 as a substrate for detecting keyboard switches. In this case, although the ratio of cost increase is lower than a case of adopting a double-sided through-hole type substrate, it would not change the fact of being a considerable cost increase compared to the single substrate in FIG. 9.